Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Elias Stefanakos, Ph.D.

Committee Member

Arash Takshi, Ph.D.

Committee Member

Sylvia Thomas, Ph.D.

Committee Member

Ashok Kumar, Ph.D.

Committee Member

Sesha Srinivasan, Ph.D.

Keywords

Asymmetric Electrode, Composite Gel, Electrochromism, Polyaniline, Touchchromism

Abstract

Electrochromic devices (ECDs) have triggered great interest because of their potential applicability in energy-efficient buildings and low power display systems, including reflective type smart windows/mirrors and wearable-flexible devices. In the past decades, electrochromic technologies with different device structures and materials have been proposed. The idea of employing a simple device structure with a durable, cost effective electrolyte is crucial to designing and manufacturing high-performance ECDs. With this idea in mind, this thesis describes the various efforts to develop a simple ECD comprising of a composite single active layer gel electrolyte, sandwiched between two transparent conducting electrodes, lasting over 10,000 cycles with low power consumption. The research evolved through the development and testing of various ECD structures.

At first, a polymer composite dye-thin-film coated on a conducting substrate enabled a reversible color change from dark to transparent when the thin film was touched (touchchromic device) by a specific metal, without any other external excitation. The results showed that the coloration and decoloration depend upon the composition of the electrolyte, type of metals, film thickness, and the nature of the composite film. As simple as this idea may be, it turned out difficult to apply it to commercial devices.

In the second approach, a simple and potentially inexpensive electrochromic device was made, consisting of a single active composite gel layer placed between two transparent conducting fluorine-doped tin oxide glass plates. The single active layer gel electrolyte is a mixture of conducting polymer (polyaniline), polyvinyl alcohol (PVA), acid, oxidant, and synthetic dye. The results showed color change in the ECDs from dark blue to transparent at an applied potential in the range of +1.5 V to – 1.5 V. The effects are explained in terms of the change in the chemical structure of the conducting polymer and synthetic dye at different bias voltages.

In the third approach, to obtain different colors, the gel was modified by adding different types of synthetic dyes to the single active PANI composite gel electrolyte. Polyvinyl alcohol, polyaniline, and ammonium persulfate intercalated with distinct synthetic dyes resulted in enhanced transmittance modulation and diverse colors at the oxidation and reduction peaks. The results showed that the redox characteristics of the dye greatly influence the overall oxidation and reduction potentials of the EC device's active gel layer.

Even though the single (all-in-one) active layer EC device is very attractive, for the case of water-based electrolytes it has exhibited a short lifecycle due to bubble formation when the applied voltage exceeds 1.23 volts (water electrolysis). This led to the concept of an asymmetric EC device. In the asymmetric electrode structure one of the fluorine-doped tin oxide electrodes was coated by a gold (Au) thin film. The results showed that asymmetric ECDs (two electrodes having different work functions) can operate within the smaller voltage window of -1 volt to +1 volt, requiring reduced operating power and overcoming the potential problem of bubble formation in the active ECD layer.

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